Small arms cartridge

ABSTRACT

Provided is a centerfire rifle cartridge. A case has a head end, an open mouth end for receiving a projectile, and a central axis. The head end has a base portion substantially conforming to that of a .50 BMG cartridge. The case has a case wall with a taper of up to 1.0° relative to the central axis. A frusto-conical shoulder portion extends from the case wall and has an angle of approximately 40° relative to the central axis. A neck portion extends from the shoulder portion and has a mouth sized to receive a .375 caliber (9.5 mm) projectile.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application No. 62/580,476, filed Nov. 2, 2017, and incorporates the same herein by reference.

TECHNICAL FIELD

This invention relates to a firearm cartridge and barrel chamber for it. More particularly, it relates to a cartridge and cartridge load combination for efficient, high-velocity, long-range, precision shooting.

BACKGROUND

Ballistics, the science and study of projectiles and firearms, may be divided into three categories. These are: interior ballistics, exterior ballistics, and terminal ballistics. Interior ballistics relates to the propulsion of a projectile, through the bore of a barrel, to achieve a determined velocity as it leaves the muzzle of the barrel. Exterior ballistics relates to the flight of the projectile and how that flight is affected by its shape, its velocity, and external forces, including wind and gravity. Terminal ballistics relates to what effect the projectile has on the target it strikes.

A projectile may be selected for the desired effect on its intended target and the range it must travel from the firearm to engage the target. One of the most critical factors on external ballistic performance is the ballistic coefficient (BC) of the projectile. The BC of a bullet is a measure of its ability to overcome air resistance in flight. The drag coefficient of a particular bullet shape and mass will vary depending on its velocity. Propelling a bullet with a high BC at the highest achievable velocity will provide the best exterior ballistics performance, i.e., the “flattest” trajectory, for long range shooting. Muzzle velocity is determined by interior ballistics performance.

An ammunition cartridge “load” is a combination of the selected cartridge case, powder propellant charge, primer, and projectile (or bullet). The performance of a cartridge load is determined by the combination of the shape and size of the cartridge case, the powder propellant used (including burn rate and pressure), and the size, shape, and mass of the projectile. Once a projectile has been selected, several parameters influence the interior ballistics of an ammunition load. The shape and dimensions of the cartridge shell or case (relative to the interior bore dimensions of the gun chamber), the amount and characteristics of the propellant (powder) charge, the primer, and the primer vent connection to the main cartridge chamber, and barrel length are primary aspects of interior ballistics performance, for a projectile of a given mass, that determine muzzle velocity. Pressure and head space parameters are also important, and they must be balanced against the other parameters.

The cartridge case is a hollow cylinder with a head end and a projectile-receiving end. In “bottle-necked” cartridges, the cylinder includes a generally cylindrical body portion extending from the head end to a frusto-conical shoulder. The body portion has a central opening in the head end for a primer and an annular cartridge extraction groove formed adjacent the head end. The frusto-conical shoulder is integrally formed with the body and tapers from the body portion to a generally cylindrical integrally formed neck portion. The neck portion extends longitudinally from the shoulder portion and has a diameter smaller than the diameter of the body portion with an open mouth for receiving and holding the projectile.

While cartridge cases or shells theoretically may have any combination of dimensions, there are several reasons why cartridges are limited in their dimensions. The characteristics of gun powder and structural limitations on rifle actions (which define the chamber) make many extreme dimensional ratios impractical or unsafe. Likewise, changes in the case geometry can change the rate at which a powder burns and/or the chamber pressure produced in unpredictable ways. Performance is not always a predictable extrapolation from known data taken from existing cartridges and can produce unexpected results in practice that differ from theoretical or extrapolated calculations. Combinations that are possible to construct may exhibit impractical and/or unexpected characteristics. For example, they may cause extraordinary barrel wear, may produce unexpected chamber pressures, or affect the metal case in a way that shortens its useful life (number of times it can be reloaded safely).

Another limitation is the rifle action (receiver and bolt) into which the cartridge will be chambered and from which the projectile will be propelled (through the barrel bore). A bolt-rifle's action has an elongated opening aft of the chamber through which a cartridge is received. The length of this opening limits the cartridge overall length (OAL) that can be inserted. This opening length essentially defines the length of the action. The width of the action opening and bolt face diameter limits the diameter of the accepted cartridge(s). Typically, a rifle bolt can be made to fit an action length in a limited range of bolt face diameters to accommodate the base diameter of a cartridge. Thus, actions are produced in only a limited number of action groups or “classes.”

In these standard groups, a “short action” accepts a cartridge with a maximum OAL of 2.84 inches and, in standard width, a maximum diameter of 0.473 inch. This is sized to receive the 308 Winchester cartridge, for example. A “long action” closely accepts a cartridge with a maximum OAL of 3.34 inches and, in standard width, a maximum diameter of 0.473 inch. This is sized to receive the .30-'06 Springfield cartridge, for example. A “magnum action” accepts a cartridge with a maximum OAL of 3.681 inches and a maximum diameter of 0.588 inch. This is sized to optimally receive up to the .338 Lapua Magnum cartridge, for example. A “BMG action” or “50 BMG action” accepts a cartridge with a maximum OAL of 5.54 inches and a maximum diameter of 0.804 inch. This is sized to receive the .50 BMG and .416 Barrett, for example.

The .50 caliber BMG (Browning Machine Gun) was developed from John Browning's design in 1917. The original loading was an 800 grain bullet fired at a velocity of 2,650 feet per second (fps) out of a 46″ barrel. It was later loaded with a 900 grain bullet fired at 2,500 fps. In recent decades, the .50 BMG has gained popularity among civilian shooters as a 1,000+ yard target rifle. Currently, the .50 BMG is commonly loaded with a 650 grain bullet fired at 3,000 fps. It is designed to be effective past 2,000 meters or 2,500 yards.

There are several practical limitations to improving the performance of cartridge loads for the .50 BMG. First, its case has a very large powder capacity (more than 290 grains H₂O or almost 19 cm³). Less powder volume is needed with modern propellants, meaning the case may only be partially filled and only slow-burning powders can be effectively used. Empty space inside the case allows the powder to be positioned differently from one shot to the next, often resulting in velocity variations. Empty air space in a partially filled case can affect how the powder burns, a significant effect resulting from the empty space not being found at the end of the case nearest the projectile. Instead, since a rifle is likely to be fired at an angle near horizontal, the empty space could unevenly extend the length of the case, unpredictably affecting how the powder is ignited by the primer and how the burn spreads through the case.

A cartridge known as the .416 Barrett (10.6×83 mm) addressed some of these limitations. Using the .50 BMG as its parent case, it was shortened and necked-down to accept a .416 caliber bullet. This reduced the case capacity to about 200 grains H₂O (13 cm³) and allowed a 398 grain (gr) bullet to be propelled at an average of 3,150 fps from a 32-inch barrel.

There are various original design cartridges that have used a .375 caliber projectile, including the .375 Belted Rimless Nitro-Express (by Holland & Holland), the .375 Holland & Holland Magnum, and the .378 Weatherby Magnum. Since then, various cartridges loading a .375 caliber bullet have been developed from parent cartridges in the “magnum” family, such as the .375 Ruger and the .376 Steyr, and the .375 Lethal Magnum, using a belted case. Previously, a .375 caliber cartridge has not successfully been produced from a parent cartridge in the 50 BMG family.

SUMMARY OF THE INVENTION

The present inventors set out to develop a center-fire cartridge load and bullet combination that would produce a predictable shot out to at least 5,000 yards (4,572 meters). The present invention can utilize a .416 Barrett case as its parent, whose base is the same as that of a .50 BMG cartridge, reconfiguring the geometry of the case with a higher angle shoulder and a neck to seat a .375 caliber (9.5 mm) projectile having a mass in the range of 360 grains to over 400 grains, for use in a 50 BMG class action. The case configured according to one embodiment of the present invention has a shoulder angle of approximately 40° and provides an average case capacity of about 220 grains H₂0 (about 14.4 cm³) and cartridge overall length (OAL) of 4.825 inches.

The sharper 40° shoulder angle provides longer case life than shallower angles due to reduced material flow during peak pressures. With traditional 20°-30° shoulder angles, internal pressures cause the brass case to “flow” forward, resulting in thin necks and shoulder junctions. Thin spots eventually result in cracked cases that are no longer usable for reloading. The .416 Barrett cartridge, for example, has a 25° shoulder angle, while the .50 BMG cartridge has a shoulder angle of slightly under 16° (15 degrees, 44 minutes). The present cartridge has been found to be reloadable at least four times without annealing or trimming. The expectation is that it can be reloaded at least eight times, with necessary annealing and trimming procedures, without diminished performance.

With large cases, over-pressuring often results in the inability to extract fired brass from the chamber. Unlike an “Ackley Improved” 40° shoulder, which reduces body taper to maximize case capacity, the body taper in the present design may be maintained at approximately 0.9°.

The result is an advancement in the art that provides a long-range, precision centerfire rifle cartridge capable of propelling a projectile having optimum external ballistic characteristics to a higher muzzle velocity, as well as other unexpected and unpredictable benefits. The cartridge does not perform according to internal/external ballistic calculation predictions for the amount of propellant and weight/BC of the projectile. Instead, its performance mimics that of a projectile weighing 20% less. Measured muzzle velocities have averaged 3,150-3,200 fps with a 361 grain bullet from a 34 inch barrel, which stays at supersonic speed beyond 3,000 yards. A 38 inch barrel is calculated to allow the same projectile to achieve at least 3,300 fps.

Other aspects, features, benefits, and advantages of the present invention will become apparent to a person of skill in the art from the detailed description of various embodiments with reference to the accompanying drawing figures, all of which comprise part of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Like reference numerals are used to indicate like parts throughout the various drawing figures, wherein:

FIG. 1 is a side elevation view of a cartridge with seated bullet according to an embodiment of the invention, partially cut away to show cross-section of base and case wall;

FIG. 2 is a side view thereof showing dimensional specifications thereof;

FIG. 3 is a chamber reamer specification drawing (not to scale) for the cartridge embodiment; and

FIG. 4 is a side view of a projectile according to an embodiment of the invention.

DETAILED DESCRIPTION

With reference to the drawing figures, this section describes particular embodiments and their detailed construction and operation. Throughout the specification, reference to “one embodiment,” “an embodiment,” or “some embodiments” means that a particular described feature, structure, or characteristic may be included in at least one embodiment. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” or “in some embodiments” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the described features, structures, and characteristics may be combined in any suitable manner in one or more embodiments. In view of the disclosure herein, those skilled in the art will recognize that the various embodiments can be practiced without one or more of the specific details or with other methods, components, materials, or the like. In some instances, well-known structures, materials, or operations are not shown or not described in detail to avoid obscuring aspects of the embodiments.

Referring first to FIG. 1, therein is shown a firearm cartridge 10 according to one embodiment of the present invention. As found in all fixed ammunition, the cartridge 10 is generally symmetrical around a central axis and comprises a case 12, which provides generally cylindrical walls 14 axially extending from a base portion 16. The base portion 16 includes an annular exterior grove 18 that defines a rim 20 for extraction according to well-known means from a firearm chamber. The base portion 16 includes a primer pocket 22 formed in the head 24 of the base portion 16. The primer pocket 22 is connected through a flash hole 26 to a propellant-holding powder chamber 28 defined between the walls 14 of the case 12. Axially spaced from the base portion 16, the case walls 14 include a frusto-conical shoulder portion 30 at which the diameter of the case 12 is significantly reduced and transitions into a neck portion 32. The neck portion 32 holds or seats a projectile (bullet) 34.

Referring now to FIG. 2, dimensional aspects of this embodiment of the cartridge 10 are shown. The cartridge case 12 and seated projectile 34 are substantially symmetric about their longitudinal axis a. The projectile 34 for this embodiment is a .375 caliber solid bullet, which will be described in greater detail below. The case 12 includes a base portion 16 with dimensions consistent with those of a .50 BMG cartridge (and those of a .416 Barrett cartridge). The .416 Barrett case may be used as the “parent” for the case 12 of the present invention. Likewise, the .416 Barrett is derived from the case of a .50 BMG cartridge. Thus, all dimensions associated with the base portion 16 of the case 12 may be substantially the same as these other rounds. The case 12 is significantly shorter than that of the .50 BMG and differs significantly from those of the .416 Barrett. Specifically, the angle of the shoulder portion 30 is significantly steeper at about 40°. The transition zones between the case wall 14, shoulder portion 30, and neck portion 32 may be formed with a somewhat rounded radius according to the “Ackley improved” specifications. However, unlike a “full Ackley improved” modification, which generally removes any taper from the case wall 14 to maximize powder capacity, the cartridge 10 of the present invention retains a cartridge wall taper of up to 1°, with the preferred taper being at about 0.9°. The neck can be shortened, but the full length allows better adjustment of seating depths for projectiles. As will be discussed in greater detail below, the primer pocket 22 is sized to receive a .50 BMG-size primer. As previously described, the 40° shoulder angle reduces material flow of the case 12, when fired, and, as will be discussed in greater detail below, affects the propellant burn rate, chamber pressure, and projectile velocity.

Referring now also to FIG. 3, therein is shown a specification drawing for a chamber reamer to receiver the cartridge 10 of this embodiment in a barrel having a nominal .375 caliber bore. Note that the drawing of FIG. 3 is a schematic representation of dimensions and is not illustrated to scale. When a cartridge 10 according to this embodiment of the invention is fired in a chamber formed according to the reamer specifications shown in FIG. 3, the case 12 will be expanded (“fire formed”) by the internal pressure of the burning propellant and may force the above-described rounded transitions at the shoulder portion 30 into the more sharply defined dimensions of the barrel chamber. According to well-known methods, a previously fired case 12 may be resized to original specifications and reloaded.

Projectiles are commercially available in .375 caliber having a weight as low as 350 grains or as high as 452 grains. Of course, heavier projectiles have a higher ballistic coefficient, but require greater energy to achieve muzzle velocities necessary to maintain supersonic speed and stability for greater distances. A variety of .375 caliber projectiles 34, typically machined from solid copper and having a mass in the range of 350 to 400 grains, or above, may be utilized with successful results. Three example projectiles are described in greater detail below.

Example 1

A Flat Line™ .375 caliber bullet made by Warner Tool Company of North Swanzey, N.H., provides a 361 grain, solid copper bullet having a 2.20 inch overall length (OAL). This projectile includes a 16° boat tail and may be made in accordance with the invention described in U.S. Patent Application Publication No. 2016/0327380, published Nov. 10, 2016, the contents of which are hereby incorporated herein by reference. This projectile has ballistic coefficients (BC) acoustically measured in the range of 3,000 to 15,000 fps of G1BC=0.980 and G7BC=0.494. Ballistic coefficients measured by Doppler radar chronograph of G1BC=0.961 and G7BC=0.480. The minimum twist recommended for this projectile is 1:10 inches. The inventors have found a preferred twist rate of 1:8.65 inches to 1:8.75 inches. This example is illustrated in FIG. 4.

Example 2

A non-standard 400 grain, .375 caliber, solid copper bullet made by Berger Bullets LLC of Fullerton, Calif. Sample projectiles have a calculated BC greater than 1.0 (about 1.1). Warner Tool Company, referenced above, also makes a 400 grain, .375 caliber projectile that is suitable.

Example 3

A MTAC™ (match/tactical) 3.745/.3655 caliber, 402 grain, solid copper projectile made by Cutting Edge Machining Solutions Inc., d/b/a Cutting Edge Bullets, of Drifting, Pa., having a G1BC=0.990 and OAL of 2.209 inches. This projectile requires a twist of 1:7.5 inches or faster.

A primer (well-known, not shown) sized to seat in a primer pocket 22 configured for .50 BMG-size primers may be used. The geometry of the case 12 of the present invention provides maximum performance using a medium-slow burning powder, generally in the category of those designed for the .338 Lapua Magnum or .50 BMG cartridges. H50BMG or similar powders may be used. A powder designed for the .338 Lapua Magnum cartridge having a burn rate only slightly faster than powder intended for .50 BMG cartridge may provide better performance. For example, 168 grains of Reloder® 33 propellant powder by Alliant Powder of Anoka, Minn., has been used in the cartridge 10 of the present invention to propel a 361 grain projectile through a 34 inch barrel to an average muzzle velocity in the range of 3,150 to 3,200 fps. As previously described, the burn rate of propellant powders has been found to be different than predicted by the powder manufactures. Likewise, performance of the cartridge has been found to be different than predicted due to the novel case geometries and projectile caliber combinations.

The cartridge 10 is accommodated by a 50 BMG-class action and bolt, the case 12 having substantially the same base and head dimensions of a .50 BMG cartridge and a shorter OAL. Chamber pressures produced are within ranges safely accommodated by a 50 BMG-class action, bolt, and barrel.

As described above, unexpected performance has been produced using a 34 inch barrel in .375 caliber. The performance has been found to mimic that of the calculated performance of a projectile weighing 20% less. Using this adjusted performance calculation, it is expected that a 361 grain projectile could exceed 3,300 fps from a 38 inch barrel. Using a twist rate of 1:8.65 inches, the projectile stays supersonic past 3,000 yards, with the expectation of targetable flight to at least 5,000 yards.

While one or more embodiments of the present invention have been described in detail, it should be apparent that modifications and variations thereto are possible, all of which fall within the true spirit and scope of the invention. Therefore, the foregoing is intended only to be illustrative of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not intended to limit the invention to the exact construction and operation shown and described. Accordingly, all suitable modifications and equivalents may be included and considered to fall within the scope of the invention, defined by the following claim or claims. 

1. A centerfire rifle cartridge, comprising: a case having a head end, an open mouth end for receiving a projectile, a central axis, and an axial length measured along the central axis, the head end having a base portion up to 0.804 inches in diameter; the case having a case wall, the case wall having a taper of 0.85° to 1.0° relative to the central axis, a frusto-conical shoulder portion extending from the case wall and having an angle of 35° to 45° relative to the central axis, and a neck portion extending from the shoulder portion; and the neck portion having a mouth sized to receive a .375 caliber projectile.
 2. The cartridge of claim 1, wherein the case has a total axial length in the range of about 3.24 inches to about 3.29 inches.
 3. The cartridge of claim 1, wherein the wall has an axial length from the head end of about 2.50 inches.
 4. The cartridge of claim 1, wherein the case wall taper is about 0.9° relative to the central axis.
 5. The cartridge of claim 1, wherein the case capacity is greater than 200 grains H₂O.
 6. The cartridge of claim 5, wherein the case has a capacity in the range of about 210-230 grains H₂O.
 7. The cartridge of claim 1, further comprising a projectile having a weight in the range of about 350-410 grains
 8. The cartridge of claim 7, wherein the loaded overall length is at least 4.65 inches.
 9. The cartridge of claim 8, wherein the loaded overall length is about 4.825 inches. 